Packed columns are normally utilized in mass transfer operations which involve gas-liquid contact such as distillation and absorption processes. In operation, a liquid is usually introduced at the top of the column and flows over a packing material contained within the column. Usually, a gas is introduced at the bottom of the column and flows through the column countercurrently to the liquid. The packing material increases the intimacy of contact between the two phases thereby facilitating the transfer of material from the gas phase to the liquid phase or vice versa.
Several factors are involved in providing a packed column with a relatively high operating efficiency. Included among these factors are the column temperature and temperature gradient, column pressure and pressure drop, column size, the method of introducing the phases into the column, relative flow rates of the phases, the manner in which the column is packed, etc. Of particular importance is the selection of the packing material utilized. The packing material should be constructed in such a manner that it contributes minimally to the pressure drop across the column. Moreover, it should be chemically inert with respect to the phases which it contacts. It should also have high mechanical durability, be relatively nonabsorbent and impervious to the phases passing through the column, and should be heat and corrosion resistant.
Packing materials are available in many shapes, materials and sizes. Perhaps the most common shapes are rings and saddles such as Raschig rings, Pall rings, Ballast rings, Flexirings, Hy-Pak rings, Cascade Mini-Rings, Berl saddles, Intalox saddles, etc. These packing materials are made from a wide variety of metals, plastics, glass or ceramics and there are several advantages associated with the use of each type. Ceramics, for example, are resistant to many corrosive materials, have a low coefficient of expansion and generally are not otherwise effected by high temperatures. However, many of the prior art ceramic materials are relatively heavy and have a relatively low void space. Even the improved ceramic materials such as Intalox saddles weigh approximately 38 pounds per cubic foot and contain only 80% void space. DuPont Torvex cross flow described in U.S. Pat. Nos. 3,338,995 and 3,255,027 weighs 32 pounds per cubic foot and has only 75% void fraction. From an engineering standpoint, heavier materials are undesirable since they require the use of stronger support plates in a column which in turn could be a significant factor in causing column flooding (i.e., a condition wherein liquid will not flow freely through the column). Under usual conditions, stronger support plates are provided by eliminating some of the open areas in the plate and replacing these areas with metal to increase the strength of the plate. Under any circumstances, it is technically desirable for the support to have a greater open area than the packing in order for the support area not to be the critical region in the development of an undesirable flooding condition. Often times it is difficult to avoid this condition where heavy packing material is utilized. Thus, it is apparent that flooding may be correlated to the percentage of void space inherent in the packing material, i.e., as percentage of void space increases (and weight of the material decreases), the possibility of flooding decreases. For the foregoing reasons, it is frequently desirable to employ packing materials with relatively high void space.
Up until the recent past, packing materials have been designed with a relatively large surface area in order to provide a film of liquid for the gas to contact since it was believed that the most efficient mass transfer could take place in this manner. More recently, however, it has been observed that the coefficient of mass transfer from a gas to liquid droplets is 10 to 13 times greater than from a gas to a flat surface (Chemical Engineers Handbook, Perry 5th Ed., p. 18-42). As a result of these observations, packing materials have been designed to promote droplet formation. Typical of the packing materials designed to enhance droplet formation are Super Intalox saddles (Norton Company), each of which contains two scalloped edges having about 18 apexed points to facilitate droplet formation (see U.S. Pat. No. 3,232,589 to Eckert). Other trends in the design of improved packing materials are described in U.S. Pat. No. 3,493,218 to Castellucci, U.S. Pat. No. 3,167,600 to Worman, and U.S. Pat. No. 3,796,657 to Pretorius et al. Pretorius is not relevant because he stresses "uniformity" of cross section. U.S. Pat. No. 3,962,081 of Yarwood also mentions "structural uniformity" in claim 1. The effectiveness of the present invention is based on non-uniformity (i.e. internal voids). U.S. Pat. No. 3,748,828 of Lefebore involves liquid running down threads of yarn. This, of course, will not stand high temperatures. In Mr. Eckert's packing, the efficiency of the packing material is somewhat limited by the limited number of scallops available for droplet formation and the lack of sharp and clearly defined points. To be effective the scallops must be pointed down and in random packing they will be pointed up half the time. In U.S. Pat. No. 3,410,057 to Lerner, the packing material is similar to this invention, but the shape and purpose is different. The purpose of Lerner's patent is to separate liquid and gas and keep them separate, as noted in the following exerpts:
"Returns the disentrained liquid through flow channels established entirely within the confines of the porous bodies . . . " "All backflow or drainage of liquid occurs through the porous bodies . . . " "The voids . . . remain free to transmit gas flow." "A method of disentraining . . . "
The purpose of the column packing is not to keep the gas and liquid separate, but just the opposite--to contact them for better mass transfer. It is not to keep the liquid inside the porous bodies, but rather to make the liquid drip through the voids. It is not to keep voids free to transmit gas flow, but rather to cause liquid to drip through the voids. For this different purpose, a different design is necessary.
A big disadvantage of Lerner's shapes as shown in his FIG. 9 and described in column 2, line 50 (shaped so as to insure mostly point and line contact) is that the weight of the packing above will break the points. A further disadvantage (column 6, line 57--surfaces at random inclination) is that when the angle is too great, liquid flows inside the packing instead of dripping through the void and contacting the gas.
A serious disadvantage of random packing is that when a metal column heats and expands, the packing settles. When it is shut down or cools the column then contracts, crushing the packing and causing high pressure drop. In Lerner's claim 3 he mentions a gas-liquid contact device in conjunction with horizontal plates. The plates are not required in this invention.
In many scrubbers to remove small solid particulates the gas with particulates travels horizontally through a packed bed while liquid flows down. A disadvantage of previous packings is that the gas has a high pressure drop to force it through the packing. The packing of this invention has low pressure drop.
Sponge type catalyst supports have been described in Robert Clyde's U.S. Pat. Nos. 3,900,646 and 3,998,758. These are very desirable because they mix gases, but one disadvantage is that they have higher pressure drop than parallel hole types. Parallel holes do not mix gases, and if a hot spot (from platinum, for example) develops in the metal coating, it dissipates only along the hole, since ceramic is an insulator, resulting in the catalyst promoting a different reaction at the higher temperature. It would be very desirable if a catalyst support could be developed that had low pressure drop, promoted turbulence, mixed gases, and dissipated heat in a zag-zag fashion.
Schwartzwalder's U.S. Pat. No. 3,090,094 mentions that porous ceramics make good high temperature filters, but one big disadvantage is that they clog so quickly. Hot particulates from burning coal and from diesel engines could be removed if a method were found to hold more solids before clogging. A plume from an airplane results in an enemy being able to track it. Solids from diesel combustion are also thought to be carcinogenic. Heat exchangers are much less efficient when fouling occurs. It would be very desirable if a high temperature filter that held more solids and in which organic material could be burned out and inorganic material cleaned out with acid, could be devised. U.S. Pat. No. 3,948,623 of Ostly et al describes a high temperature air filter for foundries but it is made of metal and will not stand over 2000.degree. F.
U.S. Pat. No. 3,873,281 of Himes et al describes a flexible plastic filter that will not withstand high temperature or corrosive acids. A thin structurally weak fine filter can be put between two layers of a strong ceramic filter on the outside of a cylinder so it can withstand internal vacuum (during liquid filtration) or pressure (during pulse cleaning). The internal filter is thin to keep a low pressure drop.
In some cases particles are too small to be retained by a filter. Scientists at Lawrence Berkely Lab (Chem-Eng. News Mar. 20 '78, pg. 19) believe that invisible soot particles catalyze SO.sub.2 oxidation. It would be very desirable if an ESP (electrostatic precipitator) could be devised with more area than a flat plate.
Mr. Karl Springer of South West Research Institute reported June 27, 1978 at the Air Pollution conference in Houston that glass fibers can remove Diesel soot. To have an entire filter of such fibers, however might cause excessive pressure drop because particulates are very small (about 1/2 micron). In a composite filter of this invention the tortuous path of the gases will impinge solids on the fibers without a high pressure drop. By spreading out the solids in this "in depth" filtration, they don't block flow of the gases.
Several U.S. Pat. Nos. 3,893,917, 3,962,081, 4,052,198, 4,081,371, 4,024,056, 4,056,586, 4,024,212, and 3,947,363 for filtering molten aluminum have been assigned to Swiss Aluminum. In each case, however, a flat sponge is used. A wave form top however provides more area for filtration, and moreover, rapid clogging is avoided and the useful life extended. In this embodiment although solids may coat the glass or ceramic fibers and fill a substantial portion of the trough, the crest zone would still function. If this amount of solids were put on a flat sponge it would clog quickly. U.S. Pat. No. 4,052,198, claim 5, mentions a coarse filter and a fine filter but does not mention the great advantage of exposing more area as shown in our FIG. 7, nor the advantage of having the principal filter in between as in FIG. 15, in which the sponge is merely the support for the filter. The main problem is Diesel exhaust or coal particulates is to remove very small particles (1 or 2 microns). For this a fine filter is required. To prevent high pressure drop it must be thin and for structural strength it needs support. Our embodiment involves internal passageways. U.S. Pat. No. 4,056,586 mentions a curved plate and hollow cylinder, claims 7 and 8. Our FIG. 1 is more than a simple curved plate.
As described in our FIG. 15 it is necessary to have two concentric cylinders (or one with internal passageways) one to support the filter during operation and one to support while back-washing or blowing.
U.S. Pat. No. 4,007,923, Chia, shows a filter with slightly more area than a plain flat plate. However, Chia does not have the applicant's unique feature of undulating walls with varying thickness, the thickness in the trough being thinner than in the crest. When the material filters down from the top, the thinner section of the sponge in the trough offers less resistance allowing the fluid to flow freely through the trough and depositing solids therein. Thus, when the trough is partly filled, the fluid will again take the path of least resistance and go through the side of the sponge just above the solids, leaving the solids to build up in the trough. In this manner, the filter will clog only when the trough is completely full. Chia does not offer this effect because when the solids start to form on the plain flat plates of the filter, there will be immediate resistance, thus the liquid will not be able to flow freely as in applicant's design.
U.S. Pat. No. 3,963,504, Lundsager, states in pertinent part in claim 1 that " . . . uniform channels of essentially rectangular cross section . . . varying in size from 0.025 to 0.20 inches . . . ". Although in applicant's FIGS. 1, 4, and 15, the internal voids are uniform; they are not rectangular. Further, in all cases the internal voids are greater than one-fifth of an inch. Moreover, in all embodiments, other than FIGS. 1, 4, and 15, it will be noted that the internal voids are irregular.
U.S. Pat. No. 3,911,070, another Lundsager invention, uniform channels are formed by extrusion. When the uniform channels are formed by extrusion, one cannot readily make a sponge body with tapered channels or make the outside dimensions in a shape of a cone rather than a cylinder. Thus, in applicant's device, a larger volume of gas can be accommodated. Moreover, the platinum used in the automobile catalytic converter causes an exothermic reaction which requires a larger volume. Thus, applicant's device is able to accommodate this larger volume of gases which ordinary devices could not.
U.S. Pat. No. 3,533,753, Berger, utilizes parallel channels as in claim 10 or intersecting channels as in claim 11. In either case, the gas must make a 90 degree turn, which means considerably more pressure drop than in applicant's design. The increased pressure drop means that more force is needed in order to send the gas through the catalytic converter. Thus, applicant's design is more efficient than that of Berger.